1. Field of the Invention
The present invention relates to a microwave dielectric ceramic composition, and in particular, to an improved microwave dielectric ceramic composition which is applicable as an antenna or microwave circuit used in a band pass filter or a microwave region based on a high dielectric constant .epsilon..sub.r and quality factor Q and by controlling a temperature coefficient of a resonant frequency .tau..sub.f.
2. Description of the Conventional Art
Recently, since a communication unit such as a vehicle-mounted telephone, a cellular phone, a codeless phone, a satellite broadcast receiver, etc. is widely used, the use of a dielectric ceramic is increased as an antenna or microwave circuit used in a band pass filter or a microwave region.
The dielectric ceramic has a high dielectric constant and quality factor and a temperature coefficient of a resonant frequency .tau..sub.f defined in a range of -5 ppm/.degree. C..about.+5 ppm/.degree. C. In addition, the fabrication cost of the dielectric ceramic is needed to be preferably low. Among the dielectric ceramic compositions, (Zr,Sn)TiO.sub.4 is known to have a dielectric constant of 38. In order to fabricate the (Zr,Sn)TiO.sub.4 having a relative density of more than 90% based on a solid state reaction, a sintering temperature higher than 1600.degree. C. is required. The above-described composition is disclosed in "Chemical Processing and Microwave Characteristics of (Zr,Sn)TiO.sub.4 Microwave Dielectrics" J. Am. Ceram. Soc., 74 [6] 1320-1324 (1991) by Shinich Hirano, Takashi Hayashi, and Akiyoshi Hattori.
Therefore, in another article, an additive is disclosed, which is capable of decreasing the sintering temperature to lower than 1600.degree. C. without decreasing the microwave dielectric characteristic of the (Zr,Sn)TiO.sub.4 ceramic.
The U.S. Pat. No. 4,102,696 discloses a microwave dielectric ceramic composition in which 0.2.about.17 wt % of one selected from La.sub.2 O.sub.3, CoO, and ZnO to the composition formed of 22.about.43 wt % of TiO.sub.2, 38.about.58 wt % of ZrO.sub.2 and 9.about.26 wt % of SnO.sub.2. This composition is capable of decreasing the sintering temperature to 1320.degree. C. by adding one of La.sub.2 O.sub.3, CoO, and ZnO to the composition sintered at a temperature of 1600.degree. C.
In addition, the U.S. Pat. No. 4,665,041 discloses a dielectric ceramic obtained by sintering a mixture powder at 1350.about.1450.degree. C. which is obtained by adding one selected from ZnO less than 7 wt %, and NiO, Nb.sub.2 O.sub.5, Ta.sub.2 O.sub.5, WO.sub.3, Sb.sub.2 O.sub.5 less than 10 wt % to the composition formed of 22.about.43 wt % of TiO.sub.2, 38.about.58 wt % of ZrO.sub.2, and 9.about.26 wt % of SnO.sub.2 by less than 7 wt %.
The U.S. Pat. No. 5,132,258 discloses ceramic sintered at a temperature of 1350.about.1450.degree. C. by adding one selected from MnO, Al.sub.2 O.sub.3, CuO and Li.sub.2 O, by 5 wt % to the composition formed of xTiO.sub.2 -yZrO.sub.2 -zSnO.sub.2 wherein 0.30&lt;x&lt;0.60, 0.25&lt;y&lt;0.60, 0.025&lt;z&lt;0.20, and x+y+z=1 based on the mole ratio or adding 3 wt % of Ga.sub.2 O.sub.3 to the above-described composition or adding Nb.sub.2 O.sub.5 or Ta.sub.2 O.sub.5 by less than 10 wt % or adding ZnO by less than 5 wt %.